In the past three decades, the shortage of nurses willing to work in hospitals has been a persistent challenge in the United States. Hiring supplemental registered nurses—nurses on short-term contracts obtained through an external staffing agency—has been common to fill gaps in nurse staffing. But there has been insufficient evidence about supplemental nurse workforce trends to inform workforce policy. To address this concern, we compared qualifications and characteristics of supplemental nurses with those of permanent nurses during 1984–2008. The two groups shared similar education levels in terms of possessing a baccalaureate or higher degree. Supplemental nurses were somewhat less experienced than permanent nurses, averaging fifteen years of experience in 2008 compared to eighteen years for permanent nurses. The supplemental nurse workforce was more diverse racially and ethnically and more likely to be male than the permanent nurse workforce. These data show that employing supplemental nurses could help meet the challenges of an aging nursing workforce, the projected future shortage of nurses, and an increasingly diverse US population.